Optimized performance strategy for a multi-stage volumetric expander

ABSTRACT

A multi-stage expansion device having bypass capabilities and a variable speed drive is disclosed. In one example, the multi-stage expansion device has a housing within which a first stage, a second stage, and a third stage are housed. The housing may also be configured with internal working fluid passageways to direct a working fluid from the first stage to the second stage and/or from the second stage to the third stage. Each of the stages may include a pair of non-contacting rotors that are mechanically connected to each other and to a power output device such that energy extracted from the working fluid is converted to mechanical work at the output device. In one example, a bypass line is provided to bypass working fluid around the first stage and a bypass line is provided to bypass working fluid around the second stage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of PCT/US2015/044529, filed on Aug. 10, 2015, which claims benefit of U.S. Patent Application Ser. No. 62/043,082, filed on Aug. 28, 2014, and which applications are incorporated herein by reference. To the extent appropriate, a claim of priority is made to each of the above disclosed applications.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Contract No. DE-EE0005650 awarded by the National Energy Technology Laboratory funded by the Office of Energy Efficiency & Renewable Energy of the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

This present disclosure relates to volumetric fluid expansion devices that convert waste energy from a power plant to useful work for the purposes of increasing power plant efficiency.

BACKGROUND

Waste heat energy is necessarily produced in many processes that generate energy or convert energy into useful work, such as a power plant. Typically, such waste heat energy is released into the ambient environment. In one application, waste heat energy is generated from an internal combustion engine. Exhaust gases from the engine have a high temperature and pressure and are typically discharged into the ambient environment without any energy recovery process. Alternatively, some approaches have been introduced to recover waste energy and re-use the recovered energy in the same process or in separate processes. However, there is still demand for enhancing the efficiency of energy recovery.

SUMMARY

In one aspect of the present teachings, a multi-stage volumetric fluid expansion device is provided to generate useful work by expanding a working fluid. In one application, the volumetric fluid expansion device can be utilized to recover waste energy from a power plant, such as waste heat energy from a fuel cell or an internal combustion engine. The power plant may be provided in a vehicle or may be provided in a stationary application, such as a generator application.

The multi-stage volumetric fluid expansion device may be provided as part of a system for generating mechanical work via a closed-loop Rankine cycle. Such a system may also include a power plant that produces a waste heat stream, wherein the power plant has a waste heat outlet through which the waste heat stream exits and at least one heat exchanger in fluid communication with the waste heat stream. In operation, the heat exchanger heats the working fluid. The multi-stage fluid expansion device can be configured to generate mechanical work at an output device from the working fluid and be provided with a housing within which a first stage, a second stage, and a third stage are disposed. The first, second, and third stages can be configured to sequentially expand the working fluid and product mechanical work at the output device. A condenser may also be provided to partially or fully condense the working fluid while a pump may be provided to pump the condensed working fluid back to the heat exchanger.

The volumetric fluid expansion device can be provided with a first working fluid bypass line extending between the first inlet and first outlet of the first fluid expansion stage to allow working fluid to bypass the first stage. In one aspect, the first working fluid bypass line can include a first control valve. The volumetric fluid expansion device can be provided with a second working fluid bypass line extending between the first inlet and first outlet of the second fluid expansion stage to allow working fluid to bypass the first stage. In one aspect, the second working fluid bypass line can include a second control valve. A bypass line to bypass working fluid around the third stage may also be provided.

A variable speed drive for controlling the rotational speed of the fluid expansion device first, second, and third pairs of rotors may also be provided. In one aspect, the variable speed drive includes a motor connected to the power output device of the fluid expansion device.

The multi-stage expansion device first stage may include a first pair of non-contacting rotors disposed between a first inlet and a first outlet while the second stage may include a second pair of non-contacting rotors disposed between a second inlet and a second outlet. The third fluid expansion stage may include a third pair of non-contacting rotors disposed between a third inlet and a third outlet. In one aspect, the power output device is rotated by the first, second, and second third of rotors. In one example, the second outlet and third inlet are joined within the housing to form a continuous working fluid passageway extending between the second inlet and the third outlet. In one example, the first outlet and the second inlet are joined within the housing to form a continuous working fluid passageway extending between the first inlet and the third outlet.

In one aspect, the output device is mechanically coupled to the third stage, the second stage is mechanically coupled to the third stage, and the first stage is mechanically coupled to the second stage such that power developed by each of the first, second, and third stages is transmitted to the power output device. In one example, the first pair of rotors has twisted non-contacting lobes, wherein one of the first pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the first pair of rotors. The second and third pairs of rotors may be similarly configured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a vehicle having a volumetric fluid expansion device having features that are examples of aspects in accordance with the principles of the present teachings.

FIG. 2 is a schematic view of a first example of the volumetric fluid expansion device shown in FIG. 1.

FIG. 3 is a perspective view of a rotor suitable for use in the volumetric fluid expansion device shown in FIG. 1.

FIG. 4 is a schematic side view of a stage inlet of the fluid expansion device shown in FIG. 1.

FIG. 5 is a perspective view of an example fluid expansion device having features that are examples of aspects in accordance with the principles of the present disclosure.

FIG. 6 is a perspective view of the fluid expansion device shown in FIG. 5.

FIG. 7 is a perspective view of the fluid expansion device shown in FIG. 5.

FIG. 8 is a cut-away partial perspective view of the fluid expansion device shown in FIG. 5.

FIG. 9 is a cut-away partial perspective view of the fluid expansion device shown in FIG. 5.

FIG. 10 is a perspective view of the drivetrain of the fluid expansion device shown in FIG. 5.

DETAILED DESCRIPTION

Various examples will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various examples does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible examples for the appended claims. Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures.

Modern demands for fuel efficient vehicles and power plants have led to development of hybrid power-generation and propulsion systems. Generally, such systems combine a power-plant, such as an internal combustion engine or a fuel cell, and an electric motor to drive the vehicle. Each of the internal combustion engine and fuel cell emits high temperature exhaust as a byproduct of the power-generation cycle employed therein. The high temperature exhaust constitutes energy that is lost from the power-generation cycle, which, if recaptured, could be employed to improve efficiency of the cycle, and, therefore, of the propulsion system employing the same. Improvements in other applications are also desired, for example in marine agricultural and industries. Another example is stationary generator sets.

Referring to FIG. 1, a vehicle 10 is shown having wheels 12 for movement along an appropriate surface, such as a roadway. The vehicle 10 includes a power-generation system 14. The system 14 can include a power-plant 16 employing a power-generation cycle. The power-plant 16 can use a specified amount of oxygen, which may be part of a stream of intake air, to generate power. The power-plant 16 can also generate waste heat such in the form of a high-temperature exhaust gas in exhaust line 17 a byproduct of the power-generation cycle. In one example, the power-plant 16 can be an internal combustion (IC) engine, such as a spark-ignition or compression-ignition type which combusts a mixture of fuel and air to generate power. In one example, the power-plant 16 may be a fuel cell which converts chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.

The vehicle 10 may also include an energy recovery device, for example volumetric fluid expansion device 20, which recovers waste heat from the power-plant 16 to improve the efficiency of the power-plant 16. In one aspect, the volumetric fluid expansion device 20 can be a multi-stage fluid expansion device 20. A more detailed description of a multi-stage fluid expansion device 20 is provided in Patent Cooperation Treaty (PCT) International Application Publication Number WO 2014/117159 entitled MULTI-STAGE VOLUMETRIC FLUID EXPANSION DEVICE. WO 2014/117159 is hereby incorporated herein by reference in its entirety.

In one example, and as shown in FIG. 1, an organic Rankine cycle (ORC) can be used to power the fluid expansion device 20. In such an example, a piping system 1000 including a heat exchanger 18 can be provided that transfers heat from the exhaust gas line 17 to a working fluid 12 that can then be delivered to the volumetric fluid expansion device 20. The working fluid 12 may be a solvent or combination of solvents, such as ethanol, n-pentane, toluene, and/or water. A condenser 19 can also be provided which creates a low pressure zone for the working fluid 12 and thereby provides a location for the working fluid 12 to condense. Once condensed, the working fluid 12 can be delivered to the heat exchanger 18 via a pump 17. A more detailed description of an ORC system being utilized to drive an energy recovery device 20 is provided in Patent Cooperation Treaty (PCT) International Application Publication Number WO 2013/130774 entitled VOLUMETRIC ENERGY RECOVERY DEVICE AND SYSTEMS. WO 2013/130774 is hereby incorporated herein by reference in its entirety. Additional ORC systems are disclosed in this application, as well as in PCT Application Publication WO 2014/117152 entitled VOLUMETRIC ENERGY RECOVERY SYSTEM WITH THREE STAGE EXPANSION, the entirety of which is incorporated by reference herein. The volumetric fluid expansion device 20 may also be utilized in a direct exhaust gas heat recovery process wherein the exhaust gas is the working fluid 12, as disclosed in Patent Cooperation Treaty (PCT) Application Publication WO/2014/107407 entitled EXHAUST GAS ENERGY RECOVERY SYSTEM, the entirety of which is herein incorporated by reference in its entirety.

Housing Configurations

Referring to FIG. 2, a schematic representation of an example of a multi-stage volumetric fluid expansion device 20 in accordance with the present teachings is shown. FIGS. 5-10 show a physical example of the volumetric fluid expansion device 20. As presented, the multi-stage volumetric fluid expansion device 20 can include a first stage 20-1, a second stage 20-2, and a third stage 20-3. It should be understood that although three stages is shown, the device could be provided with fewer stages, such as two stages, or more stages, such as four, five, six, or more stages. In generalized terms, each of the stages 20-1, 20-2, 20-3 is or can be placed in fluid communication with the other such that the working fluid 12 passes sequentially through the stages 20-1, 20-2, 20-3 whereby energy from the fluid is transferred to useful work. The fluid expansion device 20 may also include a power output device 400 configured to transfer useful work from the stages 20-1, 20-2, 20-3 to a power input location of the vehicle 10 or power plant 16.

As shown, the first stage 20-1 can include a main housing 102 that defines a first working fluid passageway 106 extending between a first inlet 108 and a first outlet 110. Similarly, the second stage 20-2 can include a main housing 202 defining a working fluid passageway 206 extending between a second inlet 208 and a second outlet 210 while the third stage 20-3 can have a main housing 302 defining a working fluid passageway 306 extending between a third inlet 308 and a third outlet 306. The fluid expansion device 20 can also be provided with compartments 150, 152, 154, and 156 to house bearings, timing gears, and/or step gears, as disclosed in PCT Application Publication WO 2014/117159. In one example, the compartments 152 and 154 can be configured to provide a boundary between the working fluid pathways 106/206 and 206/306 so as to prevent the working fluid 12 from bypassing internally from the first stage 20-1 to the second stage 20-2 and from the second stage 20-2 to the third stage 20-3 outside of the defined working fluid pathways 106, 206, 306.

Disposed within each of the working fluid passageways 106, 206, 306 can be a pair of meshed rotors 130/132, 230/232, and 330/332, respectively. Each pair of meshed rotors 130/132, 230/232, and 330/332 can be configured such that the rotors are overlapping and rotate synchronously in opposite directions. As the working fluid 12 passes through the inlet 108, 208, 308, across the meshed rotors 130/132, 230/232, 330/332, and to the respective outlet 110, 210, 310, the working fluid 12 undergoes a pressure drop which imparts rotational movement onto the rotors, thus creating mechanical work that can be input back into the power plant 16. Accordingly, each inlet port 108, 208, 308 can be configured to admit the working fluid 12 at an entering pressure whereas the corresponding outlet port 110, 210, 310 can be configured to discharge the working fluid 12 at a leaving pressure lower than the entering pressure. In such a configuration, the working fluid 12 enters inlet 108 at a first pressure and leaves outlet 110 and enters inlet 208 at a second pressure lower than the first. The working fluid can then exit outlet 210 and enter inlet 308 at a third pressure lower than the second and can subsequently exit outlet 310 at a fourth pressure lower than the third. In one example, the pressure drop from the first inlet 108 to the third outlet 310 can be about 10 bar wherein the pressure drop between the first inlet and the first outlet can be about 5 bar, the pressure drop between the second inlet 208 and the second outlet 210 can be about 3 bar, and the pressure drop between the third inlet 308 and the third outlet 310 can be about 2 bar.

With reference to the example shown in FIG. 2, the housings 102, 202, 302 can be configured such that the first outlet 10 and the second inlet 208 can be formed as a common internal working fluid passageway, as can be the second outlet 210 and the third inlet 208. In this configuration, as the working fluid 12 enters the first stage 20-1, at the first inlet 108, the working fluid 12 stays entirely internal to the fluid expansion device 20 until reaching the third outlet 310. By creating an entirely internal working fluid passageway 106/206/306 between the first inlet 108 and the third outlet 310, the potential leak paths for working fluid are even further reduced, which in turn also reduces pressure drop losses and packaging complexity.

Bypass Configurations

Still referring to FIG. 2, the volumetric energy recovery device can be provided with one or more bypass lines to allow some or all of the working fluid 12 to bypass one or more of the expander stages 20-1, 20-2, 20-3. Bypassing stages of the expansion device 20 enables the operation of the expansion device 20 to be optimized in light of actual system operating conditions. In one example, bypassing can be effective during transient operating conditions to accommodate the time lag between system activation and the working fluid 12 actually being sufficiently heated (e.g. superheated) in the heat exchanger 18. The bypassing of stages also allows for the same expander 20 to be operated with different working fluids 12. In some applications, the internal gearing ratios between the stages 20-1, 20-2, 20-3 is a function of the expansion ratio of a particular working fluid to be used in the expander 20. As different working fluids 12 can have different expansion ratios, an expander 20 may run optimally for one working fluid, but not another. For example, if water were to be used as a working fluid in an expansion device 20 that is optimized for an ethanol based working fluid, the internal gearing could be too low resulting in a first stage displacement that is too small for the expansion device 20 to be operated properly. In such an instance, the pressure drop across the first and second expander stages 20-1 and 20-1 could be high enough such that no pressure is available for the third stage 20-3. Where the third stage 20-3 is designed as the largest displacement size and is responsible for the majority of the power generated by the expansion device 20, such an operating condition would drastically decrease the overall performance of the expansion device 20. However, if the first and/or second stages are bypassed such that it is ensured that the third stage 20-1 receives the working fluid 12 at an optimal pressure, the power output of the third stage 20-3 can be maximized to an extent that much of the otherwise lost power output can be realized at the expander output device 400.

In one example, a first bypass line 51 can be provided that places the first stage inlet 108 in fluid communication with the first stage outlet 110. Likewise, a second bypass line 53 can be provided that places the second stage inlet 208 (or first stage outlet 110) in fluid communication with the second stage outlet 210 (or the third stage inlet 308). In one example, a bypass line 55 is provided that places the third stage inlet 308 (or second stage outlet 210) in fluid communication with the third stage outlet 310. Accordingly, the first bypass line 51 allows for the working fluid to bypass around the first stage 20-1, the second bypass line 53 allows for the working fluid to bypass around the second stage 20-2, and the third bypass line 55 allows for the working fluid to bypass around the second stage 20-3.

Referring to FIGS. 5-9, it can be seen that a bypass line connection tube 60 can be provided at the inlet of the second stage 20-2 and that a bypass line connection tube 62 can be provided at the inlet of the third stage 20-3. The bypass line connection tube 60 can be configured to connect to bypass lines 51 and/or 53 via a mechanical connector, for example via a SWAGELOK® type tubing connector. Similarly, the bypass line connection tube 62 can be configured to connect to bypass line 55 via a mechanical connector, for example via a SWAGELOK® type tubing connector. As shown, the bypass line connection tube 60 extends through the second stage housing 202 and into fluid passageway 206 while the bypass line connection tube 62 extends through the third stage housing 302 and into fluid passageway 306. In one example, the angle of the bypass line connection tubes 60, 62 with respect to the housing is arranged to optimally introduce the bypassed working fluid 12 into the respective rotors.

In one aspect, desired flow through the bypass lines 51, 53, 55 can be controlled through the operation of one or more control valves. For example, bypass line 51 can be provided with a control valve 52, bypass line 53 can be provided with a control valve 54, and bypass line 55 can be provided with a control valve 56. It should be understood that more or fewer valves or different types of valves may be provided. For example, a three way valve could be utilized in lieu of the two valves shown for the first and second bypass lines 51, 53. In one example, valves 52, 54, and 56 are automatically controlled ball-type control valves.

In the exemplary configuration shown, the bypass lines and valves do not prevent flow through a particular stage 20-1, 20-2, 20-3, but rather provide a lower pressure drop pathway around the bypassed stage while allowing flow through each stage to be open. Accordingly, when flow is allowed through a bypass line, some of the working fluid 12 will still travel through the expander stage being bypassed although at a much lower volume and pressure drop. The degree to which the working fluid 12 passes through the bypass line instead of the expander stage can be controlled by the valve(s) itself and by the size of the bypass line. Alternatively, the bypass lines and valves can be configured to actively block flow through the stage being bypassed such that all of the working fluid 12 is directed around the stage being bypassed.

In the configuration shown, the bypass valves 52, 54, 56 can be opened and closed (and/or modulated) to bypass any single stage or combination of stages to achieve a desired bypass result. For example, where the first stage bypass valve 52 is open and the second and third stage bypass valves 54, 56 are closed, at least some of the working fluid 12 will be bypassed around the first stage 20-1, but not bypassed around the second and third stages 20-2, 20-3. Where the first and second bypass valves 52, 54 are open and the third stage bypass valve 56 is closed, at least some of the working fluid 12 will be bypassed around both the first and second stages 20-1, 20-2, but not bypassed around the third stage 20-3. The bypass valves 52, 54, 56 can be further operated to bypass only the second stage 20-2, to bypass only the third stage 20-3, to bypass the second and third stages 20-2, 20-3, and to bypass the first and third stages 20-1, 20-3.

Variable Speed Drive System

The expansion device 20 can also be provided with a variable speed drive 58 to control the rotational speed of the expansion device rotors via an output shaft, for example through the power output device 400. In one aspect, the variable speed drive 58 includes a motor and a controller to vary the speed of the motor. The variable speed drive 58 can be further configured to act as a generator when the power output of the fluid expansion device 20 is sufficient. The variable speed drive 58 can be used to replace the connection between the power output device 400 and the power plant 16 which typically fixes the speed ratio between the power plant 16 crankshaft and the fluid expansion device 20. As such, the utilization of the variable speed drive 58 can decouple the power plant operating speed from the power output device 400 to result in more efficient operation of the fluid expansion device 20. As with the bypass lines and valves, the variable speed drive 58 can be configured and operated to optimize the operation of the fluid expansion device to accommodate different types of working fluids and/or varying operating conditions of the system.

Electronic Control System

As described above, the expansion device 20 can be placed in various bypass operational modes. An electronic control system can be provided that monitors, initiates, and controls the initiation of the various modes. In one example, an electronic controller 50 monitors various sensors and operating parameters of the expansion device 20 and/or the vehicle power plant 16 to configure the expansion device 20 into the most appropriate bypass mode of operation such that power output of the expansion device 20 is optimized.

Referring to FIG. 2, the electronic controller 50 is schematically shown as including a processor 50A and a non-transient storage medium or memory 50B, such as RAM, flash drive or a hard drive. Memory 50B is for storing executable code, the operating parameters, and potential inputs from an operator interface, while processor 50A is for executing the code. Electronic controller 50 is configured to be connected to a number of inputs and outputs that may be used for implementing the bypass operational modes. For example, the electronic controller 50 can receive information from a vehicle control area network (CAN) bus 56 and information from sensors associated with the expansion device 20 (e.g. mass flow rate sensors, pressure sensors, temperature sensors, etc.). One skilled in the art will understand that many other inputs are possible.

Examples of outputs from the controller 50 are outputs for the operation of the control valves 52, 54, and 56 and for the operation of the variable speed drive 58. Other outputs are possible as well. In one embodiment, the electronic controller 50 is configured to include all required operational outputs for the operation of the fluid expansion device 20.

The electronic controller 50 may also include a number of maps or algorithms to correlate the inputs and outputs of the controller 50. For example, the controller 50 may include an algorithm to control the position of the valves 52, 54, and 56 based on the inlet conditions from the ORC system engine speed, and expansion device speed to achieve a desired mass flow rate through the expansion device stages 20-1, 20-2, and 20. The electronic controller 50 may also store a number of predefined and/or configurable parameters and offsets for determining when each of the modes is to be initiated and/or terminated. As used herein, the term “configurable” refers to a parameter or offset value that can either be selected in the controller (i.e. via a dipswitch) or that can be adjusted within the controller.

Rotor Configurations

In one example, each of the rotors 130, 132, 230, 232, 330, and 332 (collectively referred to as rotors 30, 32) can be attached to a respective rotor shaft 138, 140, 238, 240, 338, and 340 (collectively referred to as rotor shafts 38, 40). The rotor shafts 38, 40 can be rigidly connected to the rotors 30, 32 and thus rotate as the rotors are rotated. The rotor shafts 138, 238, 338 can be individual separate shafts rotationally connected through gear sets (e.g. step-up gear sets, step-down gear sets, one-to-one gear sets, etc.) or form part of a common shaft 38. Likewise, rotor shafts 140, 240, and 340 can be individual separate shafts or form part of a common shaft 38.

Each of the rotors 130/132, 230/232, 330/332, collectively referred to as rotors 30, 32 in this section and with reference to FIGS. 3-4, can provided with a plurality of lobes. As shown in FIG. 3, each rotor 30, 32 can be provided with three lobes, 30-1, 30-2, 30-3 in the case of the rotor 30, and 32-1, 32-2, 32-3 in the case of the rotor 32. Although three lobes are shown for each rotor 30 and 32, each of the two rotors may have any number of lobes that is equal to or greater than two. For example, PCT Publication WO 2013/130774 shows a suitable rotor having four lobes. Additionally, the rotors of one or more of the stages 20-1, 20-2, 20-3 may have a different number of lobes than the rotors of the other stages 20-1, 20-2, 20-3 in the device 20.

In one example, the number of lobes can be the same for each rotor 30 and 32. This is in contrast to the construction of typical rotary screw devices and other similarly configured rotating equipment which have a dissimilar number of lobes (e.g. a male rotor with “n” lobes and a female rotor with “n+1” lobes). Furthermore, one of the distinguishing features of the expansion device 20 is that the rotors 30 and 32 are identical, wherein the rotors 30, 32 are oppositely arranged so that, as viewed from one axial end, the lobes of one rotor are twisted clockwise while the lobes of the meshing rotor are twisted counter-clockwise. Accordingly, when one lobe of the rotor 30, such as the lobe 30-1 is leading with respect to the inlet port 24, a lobe of the rotor 32, such as the lobe 30-2, is trailing with respect to the inlet port 24, and, therefore with respect to a stream of the high-pressure fluid 12.

As previously mentioned, the first and second rotors 30 and 32 can be interleaved and continuously meshed for unitary rotation with each other. In one example, the lobes of each rotor 30, 32 are twisted or helically disposed along the length L of the rotors 30, 32. In one example, each rotor 30, 32 has straight lobes along the length L of the rotors 30, 32. Upon rotation of the rotors 30, 32, the lobes at least partially seal the fluid 12 against an interior side of the housing at which point expansion of the fluid 12 only occurs to the extent allowed by leakage which represents and inefficiency in the system. In contrast to some expansion devices that change the volume of the fluid when the fluid is sealed, the volume defined between the lobes and the interior side 33 of the housing is constant as the fluid 12 traverses the length of the rotors 30, 32. Accordingly, the expansion device 20 can be referred to as a “volumetric device” as the sealed or partially sealed fluid volume does not change wherein the working fluid 12 is generally not reduced or compressed.

In operation, the rotor shafts 38, 40 can be rotated by the working fluid 12 as the fluid undergoes expansion from the higher first pressure working fluid 12 to the lower second pressure working fluid 12. Accordingly, the shafts 38, 40 are configured to capture the work or power generated by the expansion device 20 during the expansion of the fluid 12 that takes place between the inlet port 108, 208, 308 and the respective outlet port 110, 210, 310. As discussed previously, the work is transferred from the shafts 38, 40 as output torque from the expansion device 20 via output device 400.

In one aspect of the geometry of the expansion device 20, each of the rotor lobes 30-1 to 30-3 and 32-1 to 32-3 has a lobe geometry in which the twist of each of the first and second rotors 30 and 32 is constant along their substantially matching length L. Alternatively, the lobes 130, 132, 230, 232, 330, 332 can be provided without a twist although a drop in efficiency would be expected to occur. In one example, lobes 130, 132 are provided as straight lobes while lobes 230, 232, 330, 332 are provided as twisted lobes. In one example, the length L of all rotors 130, 132, 230, 232, 330, 332 is the same. In one example, the length L of the rotors 130, 132 is less than a length L of the rotors 230, 232, which is in turn less than a Length L of the rotors 330, 332.

The various examples described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the examples and applications illustrated and described herein, and without departing from the true spirit and scope of the disclosure. 

What is claimed is:
 1. A multi-stage volumetric fluid expansion device comprising: a. a first fluid expansion stage having a first pair of non-contacting rotors disposed between a first inlet and a first outlet, the first fluid expansion stage being configured to generate useful work at the first pair of rotors by expanding a working fluid from a first pressure to a second pressure that is lower than the first pressure; b. a second fluid expansion stage having a second pair of non-contacting rotors disposed between a second inlet and a second outlet, the second fluid expansion stage being configured to generate useful work at the second pair of rotors by receiving the working fluid from the first fluid expansion stage outlet and expanding the working fluid to a third pressure that is lower than the second pressure; c. a third fluid expansion stage having a third pair of non-contacting rotors disposed between a third inlet and a third outlet, the third fluid expansion stage being configured to generate useful work at third pair of rotors by receiving the working fluid from the second fluid expansion stage outlet and expanding the working fluid to a fourth pressure that is lower than the third pressure; d. a first working fluid bypass line extending between the first inlet and first outlet of the first fluid expansion stage, the first working fluid bypass line including a first control valve; e. a second working fluid bypass line extending between the first inlet and first outlet of the second fluid expansion stage, the second working fluid bypass line including a second control valve; f. a power output device rotated by the first, second, and second third of rotors.
 2. The multi-stage volumetric fluid expansion device of claim 1, further comprising: a. a variable speed drive for controlling the rotational speed of the fluid expansion device first, second, and third pairs of rotors, the variable speed drive including a motor connected to the power output device.
 3. The multi-stage volumetric fluid expansion device of claim 1, further comprising: a. a housing within which the first, second, and third pairs of rotors is disposed, wherein the second outlet and third inlet are joined within the housing to form a continuous working fluid passageway extending between the second inlet and the third outlet, wherein the first bypass line includes a bypass connection tube extending through the housing and into the passageway, wherein the second bypass line includes a bypass connection tube extending through the housing and into the passageway
 4. The multi-stage volumetric fluid expansion device of claim 1, wherein: a. the first outlet and the second inlet are joined within the housing to form a continuous working fluid passageway extending between the first inlet and the third outlet.
 5. The multi-stage volumetric fluid expansion device of claim 1, wherein: a. the first pair of rotors have twisted non-contacting lobes, wherein one of the first pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the first pair of rotors; b. the second pair of rotors have twisted non-contacting lobes, wherein one of the second pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the second pair of rotors; and c. the third pair of rotors have twisted non-contacting lobes, wherein one of the third pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the third pair of rotors.
 6. A system for generating mechanical work via a closed-loop Rankine cycle, the system comprising: a. a power plant that produces a waste heat stream, wherein the power plant has a waste heat outlet through which the waste heat stream exits; b. at least one heat exchanger in fluid communication with the waste heat stream, the heat exchanger being configured to heat a working fluid; c. a multi-stage fluid expansion device configured to generate mechanical work at an output device from the working fluid, the expansion device having a housing within which a first stage and a second stage are disposed, the first stage being configured to expand the working fluid, the second stage being configured to receive the working fluid from the first stage and to expand the working fluid; d. a condenser constructed and arranged to condense the working fluid; e. a pump constructed and arranged to pump the condensed working fluid to the at least one heat exchanger; and f. a first working fluid bypass line arranged to bypass at least a portion of the working fluid around the first stage and to the second stage, the first working fluid bypass line including a first control valve.
 7. The system for generating mechanical work of claim 6, wherein the multi-stage fluid expansion device housing further includes: a. a third stage disposed within the housing that is configured to receive the working fluid from the second stage and to expand the working; b. a second working fluid bypass line arranged to bypass at least a portion of the working fluid around the second stage and to the third stage, the second working fluid bypass line including a second control valve.
 8. The system for generating mechanical work of claim 7, wherein: a. The housing defines an internal working fluid pathway within which the working fluid can pass internally from the first stage to the second stage and from the second stage to the third stage.
 9. The system for generating mechanical work of claim 7, further comprising: a. a variable speed drive for controlling the rotational speed of the fluid expansion device first, second, and third pairs of rotors, the variable speed drive including a motor connected to the power output device.
 10. The system for generating mechanical work of claim 7, further comprising: a. a housing within which the first, second, and third pairs of rotors is disposed, wherein the second outlet and third inlet are joined within the housing to form a continuous internal working fluid passageway extending between the second inlet and the third outlet, wherein the first bypass line includes a bypass connection tube extending through the housing and into the passageway, wherein the second bypass line includes a bypass connection tube extending through the housing and into the passageway
 11. The multi-stage volumetric fluid expansion device of claim 8, wherein: a. the first pair of rotors have twisted non-contacting lobes, wherein one of the first pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the first pair of rotors; b. the second pair of rotors have twisted non-contacting lobes, wherein one of the second pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the second pair of rotors; and c. the third pair of rotors have twisted non-contacting lobes, wherein one of the third pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the third pair of rotors. 